School of Chemistry, National University of Ireland, Galway, Ireland.
J Am Soc Mass Spectrom. 2011 Jun;22(6):1052-62. doi: 10.1007/s13361-011-0102-3. Epub 2011 Mar 31.
Collision induced dissociation (CID) fragmentation was compared between reducing and reduced sulfated, sialylated, and neutral O-linked oligosaccharides. It was found that fragmentation of the M - H ions of aldoses with acidic residues gave unique Z-fragmentation of the reducing end GalNAc containing the acidic C-6 branch, where the entire C-3 branch was lost. This fragmentation pathway, which is not seen in the alditols, showed that the process involved charge remote fragmentation catalyzed by a reducing end acidic anomeric proton. With structures containing sialic acid on both the C-3 and C-6 branch, the M - H ions were dominated by the loss of sialic acid. This fragmentation pathway was also pronounced in the M - 2H ions revealing both the C-6 Z-fragment plus its complementary C-3 C-fragment in addition to glycosidic and cross ring fragmentation. This generation of the Z/C-fragment pairs from GalNAc showed that the charges were not participating in their generation. Fragmentation of neutral aldoses showed pronounced Z-fragmentation believed to be generated by proton migration from the C-6 branch to the negatively charged GalNAc residue followed by charge remote fragmentation similar to the acidic oligosaccharides. In addition, A-type fragments generated by charge induced fragmentation of neutral oligosaccharides were observed when the charge migrated from C-1 of the GalNAc to the GlcNAc residue followed by rearrangement to accommodate the (0,2)A-fragmentation. LC-MS also showed that O-linked aldoses existed as interchangeable α/β pyranose anomers, in addition to a third isomer (25% of the total free aldose) believed to be the furanose form.
比较了还原和还原硫酸化、唾液酸化和中性 O-连接寡糖的碰撞诱导解离 (CID) 碎片。结果发现，带有酸性残基的醛糖 M - H 离子的碎裂给出了含有酸性 C-6 支链的还原端 GalNAc 的独特 Z- 碎裂，其中整个 C-3 支链丢失。这种在 alditols 中未见的碎裂途径表明，该过程涉及由还原端酸性端基质子远程催化的电荷碎裂。对于在 C-3 和 C-6 支链上都含有唾液酸的结构，M - H 离子主要通过唾液酸的丢失。这种碎裂途径在 M - 2H 离子中也很明显，除了糖苷和交叉环碎裂外，还揭示了 C-6 Z- 片段及其互补的 C-3 C- 片段。GalNAc 生成的 Z/C- 片段对表明电荷未参与其生成。中性 aldoses 的碎裂显示出明显的 Z- 碎裂，据信是通过质子从 C-6 支链迁移到带负电荷的 GalNAc 残基产生的，然后是类似于酸性寡糖的远程电荷碎裂。此外，当电荷从 GalNAc 的 C-1 迁移到 GlcNAc 残基并随后重排以适应 (0,2)A- 碎裂时，观察到中性寡糖电荷诱导碎裂产生的 A 型片段。LC-MS 还表明，O-连接的 aldoses 除了第三种异构体（总游离 aldose 的 25%）外，还以可互换的α/β吡喃糖互变异构体形式存在，被认为是呋喃糖形式。
